EP2607396B1

EP2607396B1 - Composition for injection molding, sintered compact, and method for producing sintered compact

Info

Publication number: EP2607396B1
Application number: EP12194386.4A
Authority: EP
Inventors: Nobuyuki Hamakura; Hidefumi Nakamura
Original assignee: Seiko Epson Corp
Current assignee: Seiko Epson Corp
Priority date: 2011-11-30
Filing date: 2012-11-27
Publication date: 2014-12-24
Anticipated expiration: 2032-11-27
Also published as: US20150376397A1; JP2013112886A; JP5857688B2; EP2607396A1; US9249292B2

Description

BACKGROUND

1. Technical Field

The present invention relates to a composition for injection molding, a sintered compact, and a method for producing a sintered compact.

2. Related Art

A powder metallurgy process for producing a metal product by sintering a molded body containing a metal powder has been widely used in many industrial fields recently because a near net shape sintered compact can be obtained using the process. Further, a ceramic powder is also used in place of a metal powder.
There are a lot of methods for producing a molded body (molding methods), however, a powder injection molding method in which an inorganic powder and an organic binder are mixed and kneaded, and injection molding is performed using the resulting kneaded material (compound) is known. A molded body produced by such a powder injection molding method is then subjected to a degreasing treatment to remove the organic binder, followed by firing, whereby a metal product or a ceramic product in a desired shape is obtained.
In such a powder injection molding method, it is necessary to select an appropriate organic binder according to various purposes, for example, for the purpose of imparting shape retainability to the molded body.
For example, JP-A-2008-75153 discloses, as an organic binder to be used in a powder injection molding method, a polyacetal resin, a polystyrene, a polyolefin, a higher fatty acid, etc.
US 6051184 A discloses a metal powder injection moldable composition which hardly causes debinding deformation. This composition consists of a metal powder and an organic binder. The components which constitute the organic binder are: a. polyoxymethylene having a Vicat softening temperature A ≧ 150 °C; b. polypropylene having a Vicat softening temperature B≧130 °C; c. an organic compound whose viscosity at said Vicat softening temperature A not higher than said temperature B; d. a thermoplastic resin whose Vicat softening temperature is not higher than said temperature B.
WO 2010/106949 A1 discloses an injection molding composition which has high thermal decomposability and rarely suffers from degreasing deformation when heated. The injection molding composition is capable of providing a good degreased body in a short time even in cases where a molded body has a large thickness. Specifically disclosed is an injection molding composition which is composed of a sinterable metal powder and an organic binder. The injection molding composition is characterized in that the organic binder is composed of (a) apolylacticacid, (b) a polyoxymethylene, (c) a polypropylene, (d) an organic compound having a viscosity at 150°C of not more than 200 mPa·s, and (e) a thermoplastic resin having a Vicat softening point of not more than 130°C.
EP 2343441 A1 discloses the preparation of a sintered variable vane such that the height of a blade unit ranges within +0.3% to +0.9%, the thickness of the blade unit ranges within - 0.6% to -0.0%, the diameter of a shaft unit ranges within +0.3% to +0.9%, the length from the lower end of the blade unit to the lower end of the shaft unit ranges within - 0-6% to -0.0% with respect to the target dimensions of a final product, and a sintering density is 95% or more of a relative density.
EP 0561343 A1 discloses an improved binder system for use in the injection molding of sinterable powders such as metal powders, ceramic powders and cermet powders comprising a (co)polymer that has a molecular weight in excess of 2000 and which contains at least one epoxy group in the molecule. The binder system comprises said (co)polymer, a (co)polymer other than said (co)polymer, and an organic compound having a molecular weight up to 2000. Using a binder system comprising (a) 3 - 80 wt% of a (co)polymer that has a molecular weight in excess of 2000 and which has at least one epoxy group in the molecule, (b) up to 70 wt% of a (co)polymer other than component (a), and (c) 20 - 80 wt% of an organic compound having a molecular weight of not more than 2000, there is provided a composition for the injection molding of sinterable powders. The compound is said to have excellent moldability and strength properties. The powders to be mixed with the binder system for use in injection molding have preferably average particle sizes in the range of 0.01 - 1000 µm.
US 5695697 A discloses a process for producing improved shaped sintered articles by first shaping a) a mixture of a ceramic or metallic powder or mixtures thereof with a moldable thermoplastic composition containing b) a thermoplastic polyoxymethylene binder and c) a second moldable and essentially inert thermoplastic polymer having a melting point between 90 and 220°C. The binder is then removed from the shaped article by exposure to a gaseous acid-containing atmosphere, preferably below its softening temperature, while the second inert thermoplastic polymer is retained as a source of elemental carbon in which the ceramic or metallic powder is finely dispersed. This retained polymer is then pyrolyzed under an inert gas at elevated temperatures of at least 600°C for conversion to a finely dispersed elemental carbon. The resulting pyrolyzed preformed product is then sintered to obtain the desired high density ceramic and/or metallic article as the final product in which elemental carbon is uniformly and finely distributed. The process provides a simpler and more easily controlled method of incorporating elemental carbon into the ceramic/metallic shaped and sintered product while also carefully controlling the initial shaping step to prevent any formaldehyde induced crosslinking in the thermoplastic binder composition.
US 5362791 A discloses thermoplastic compositions useful for producing metallic moldings containing A) 40-70% by volume of a sinterable pulverulent metal or of a pulverulent metal alloy or of a mixture thereof, B) 30-60% by volume of a mixture of B1) 50-100% by weight of a polyoxymethylene homopolymer or copolymer and B2) 0-50% by weight of a polymer which is immiscible with B1) and can be thermally removed without leaving a residue, or a mixture of such polymers, as binder, and C) 0-5% by volume of a dispersant.
JP 2002 030305 is concerned with economically producing a sintered product containing titanium in a short time. The disclosed method includes a stage in which an organic binder is added and mixed into powder for sintering composed of titanium powder or titanium alloy powder or obtained by mixing the powder of one or more kinds selected from nitride ceramics, carbide ceramics and metal powder with titanium powder to prepare a composition for compacting. The method also includes a stage in which a compact is produced by the same composition for compacting and a stage in which the produced compact is directly charged to a sintering furnace and subjected to degreasing and sintering without being subjected to a degreasing stage in an exclusively-degreasing furnace. The sintering furnace is provided with an evacuation and exhaust circuit, and a binder trap is inserted into the circuit, by which, in the temperature rising stage up to the sintering of the compact, the evacuation and exhaust circuit is operated, cracked gas in the organic binder is drawn out and removed, and further, solidified matters in wax and thermoplastic resin components contained in the cracked gas are captured in the binder trap.
JP 2006 257485 relates to the provision of a composition to be injection-molded which does not cause flaws or deformations in the resultant sintered compact even when obtained in a short degreasing time, and has more excellent injection moldability and dimensional stability than a conventionally well-known composition to be inj ection-molded. The disclosure goes on to detail that the composition to be injection-molded includes a binder and a sintering powder consisting of a metallic powder or a ceramic powder, wherein the binder comprises, by vol.%, a paraffin wax of 45 to 65%, a polyacetal resin of 0.5 to 4.5%, a modified polyolefin resin of 0.5 to 4.5%, polypropylene of 30.5 to 50% and a phthalate of 0.5 to 10%, and the binder occupies 35 to 50% of the whole composition. The phthalate is dibutyl phthalate or dioctyl phthalate.
JP 2004 232055 discloses an organic binder for producing a sintered compact with satisfactory properties free from cracks and blisters, and the provision of a resin composition for producing a sintered compact. The organic binder for producing the sintered compact comprises a polylactic acid resin as an essential component. The resin composition for producing a sintered compact is obtained by incorporating sinterable inorganic powder of 40 to 600 parts per weight to 100 parts per weight of the organic binder.
However, in the case of molding into a complicated shape, unless the shape retainability of the molded body when degreasing is sufficiently high, it is difficult to maintain the shape of the molded body when degreasing and sintering as it is immediately after molding, and therefore, deformation, chipping, or the like occurs to deteriorate the dimensional accuracy of a sintered compact. Due to this, it is necessary to further increase the shape retainability when degreasing.

SUMMARY

An advantage of some aspects of the invention is to provide a composition for injection molding capable of producing a molded body having high shape retainability when degreasing, and also capable of producing a sintered compact which is less deformed, chipped, or the like and has high quality, a sintered compact having high dimensional accuracy produced using such a composition for injection molding, and a method for producing a sintered compact capable of efficiently producing such a sintered compact.
An aspect of the invention is directed to a composition for injection molding including an inorganic powder composed of at least one of a metal material and a ceramic material and a binder containing a polyacetal-based resin and an ethylene-glycidyl methacrylate-based copolymer, wherein the polyacetal-based resin is a copolymer, wherein the ethylene-glycidyl methacrylate-based copolymer is contained in an amount of 1% by mass or more and 30% by mass or less with respect to the amount of the polyacetal-based resin.
According to this configuration, a composition for injection molding capable of producing a molded body having high shape retainability when degreasing is obtained. As a result, a composition for injection molding capable of producing a sintered compact which is less deformed, chipped, or the like and has high quality is obtained.
It is preferred that in the composition for injection molding according to the aspect of the invention, the composition has an inner layer, which is composed mainly of the ethylene-glycidyl methacrylate-based copolymer and is provided so as to cover the surface of each particle of the inorganic powder, and an outer layer, which is composed mainly of the polyacetal-based resin and is located outside the inner layer.
According to this configuration, both of the shape retainability and the moldability can be achieved.
In the composition for injection molding according to the invention, the polyacetal-based resin is a copolymer of formaldehyde and a comonomer other than formaldehyde.
According to this configuration, the shape retainability can be further enhanced.
It is preferred that in the composition for injection molding according to the aspect of the invention, the ethylene-glycidyl methacrylate-based copolymer contains at least one of vinyl acetate and methyl acrylate as a monomer constituting the copolymer.
According to this configuration, these monomers more reliably bind inorganic powder particles to one another, and therefore, the shape retainability can be particularly enhanced.
It is preferred that in the composition for injection molding according to the aspect of the invention, the softening point of the ethylene-glycidyl methacrylate-based copolymer is 65°C or higher and 105°C or lower.
According to this configuration, both of the shape retainability and the moldability can be more highly achieved.
It is preferred that in the composition for injection molding according to the aspect of the invention, a saturated fatty acid is further contained.
According to this configuration, the saturated fatty acid functions as a lubricant, and the moldability of the composition for injection molding can be further enhanced.
It is preferred that in the composition for injection molding according to the aspect of the invention, a wax is further contained.
According to this configuration, the fluidity of the composition for injection molding is increased, and the moldability is further enhanced. Further, when degreasing, the ethylene-glycidyl methacrylate-based copolymer is decomposed in a lower temperature range than the polyacetal-based resin, and therefore, as the temperature is raised, the ethylene-glycidyl methacrylate-based copolymer and the polyacetal-based resin are sequentially discharged, and as a result, the shape retainability of the molded body when degreasing can be further enhanced.
Another aspect of the invention is directed to a sintered compact, which is produced using the composition for injection molding according to the aspect of the invention described above.
According to this configuration, a sintered compact having high dimensional accuracy is obtained.
Still another aspect of the invention is directed to a method for producing a sintered compact according to claim 8, including: kneading an inorganic powder composed of at least one of a metal material and a ceramic material and a binder containing a polyacetal-based resin and an ethylene-glycidyl methacrylate-based copolymer at a temperature between the softening point of the polyacetal-based resin and a temperature lower than the softening point of the polyacetal-based resin by 10°C, thereby obtaining a kneaded material; molding the kneaded material, thereby obtaining a molded body; and degreasing the molded body, followed by firing, thereby obtaining a sintered compact.
According to this configuration, a sintered compact having high dimensional accuracy can be efficiently produced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.

FIG. 1 is a cross-sectional view schematically showing a structure of a composition for injection molding according to an embodiment of the invention.
FIG. 2 is a cross-sectional view schematically showing a structure of a composition for injection molding according to an embodiment of the invention when kneading.
FIGS. 3A and 3B are observed images of compositions for injection molding (kneaded materials) of Example 4 and Comparative Example 2, respectively.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a composition for injection molding, a sintered compact, and a method for producing a sintered compact of an embodiment of the invention will be more specifically described.

Composition for Injection Molding

The composition for injection molding of the embodiment of the invention includes an inorganic powder and a binder and is obtained by kneading these components.
The inorganic powder is composed of at least one of a metal material and a ceramic material.
The binder is a component for binding inorganic powder particles to one another, and contains a polyacetal-based resin as a component A and an ethylene-glycidyl methacrylate-based copolymer as a component B.
By the injection molding of such a composition for injection molding, a molded body which has high shape retainability also when degreasing is obtained. Therefore, by firing this molded body, a sintered compact which is less deformed, chipped, or the like and has high quality is obtained.
Hereinafter, the respective components of the composition for injection molding of the embodiment of the invention will be described in detail.

Inorganic Powder

As the inorganic powder, as described above, a powder composed of at least one of a metal material and a ceramic material is used. Specifically, other than a metal powder and a ceramic powder, a powder of a composite material of a metal material and a ceramic material, and a mixed powder of a metal powder and a ceramic powder can be exemplified.
Examples of the metal material include Mg, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Pd, Ag, In, Sn, Ta, W, an alloy of some of these metallic elements, and an alloy of any of these metallic elements with another metallic element, and among these metal materials, one metal material or a mixture of two or more metal materials is used.
Among these metal materials, particularly, a stainless steel, a die steel, a high-speed tool steel, a low-carbon steel, any of a variety of Fe-based alloys such as an Fe-Ni-based alloy, an Fe-Si-based alloy, an Fe-Co-based alloy, and an Fe-Ni-Co-based alloy, an Al-based alloy, a Ti-based alloy, or the like is preferably used. Such a metal material has excellent mechanical properties, and therefore, a sintered compact which has excellent mechanical properties and can be used in a wide range of application is obtained.
Examples of the stainless steel include SUS304, SUS316, SUS317, SUS329, SUS410, SUS430, SUS440, and SUS630.
Examples of the Al-based alloy include an aluminum simple substance and duralumin.
As the Ti-based alloy, a titanium simple substance or an alloy of titanium and a metallic element such as aluminum, vanadium, niobium, zirconium, tantalum, or molybdenum can be exemplified. Specific examples thereof include Ti-6Al-4V and Ti-6Al-7Nb. The Ti-based alloy may include a non-metallic element such as boron, carbon, nitrogen, oxygen, or silicon other than these metallic elements.
Such a metal powder may be produced by any method, but it is possible to use a metal powder produced by an atomization method (a water atomization method, a gas atomization method, a high-speed rotating water stream atomization method, etc.), a reduction method, a carbonyl method, a grinding method, or the like.
Among these metal powders, a metal powder produced by an atomization method is preferably used. According to an atomization method, it is possible to efficiently produce a metal powder having an extremely small average particle diameter as described above. In addition, it is possible to obtain a metal powder in which a variation in particle diameter is small, and the particle diameter is uniform. Therefore, when such a metal powder is used, it is possible to reliably prevent the generation of pores in a sintered compact, and it is thereby possible to improve the density.
In addition, a metal powder produced by an atomization method has a spherical shape relatively close to a perfect sphere, whereby the metal powder has excellent dispersibility and fluidity with respect to the binder. Therefore, it is possible to increase a filling property when filling a granulated powder into a mold, and eventually, it is possible to obtain a denser sintered compact.
Examples of the ceramic material include oxide-based ceramic materials such as alumina, magnesia, beryllia, zirconia, yttria, forsterite, steatite, wollastonite, mullite, cordierite, ferrite, sialon, and cerium oxide; and non-oxide-based ceramic materials such as silicon nitride, aluminum nitride, boron nitride, titanium nitride, silicon carbide, boron carbide, titanium carbide, and tungsten carbide, among these ceramic materials, one ceramic material or a mixture of two or more ceramic materials is used.
The average particle diameter of the inorganic powder to be used in the embodiment of the invention is preferably 1 µm or more and 30 µm or less, more preferably 3 µm or more and 20 µm or less, further more preferably 3 µm or more and 10 µm or less. If the inorganic powder has an average particle diameter within the above range, it is possible to eventually produce a sufficiently dense sintered compact while avoiding significant aggregation or a decrease in compressibility when molding.
If the average particle diameter is less than the above lower limit, the inorganic powder is liable to aggregate, and the compressibility when molding may be significantly deteriorated. On the other hand, if the average particle diameter exceeds the above upper limit, an interspace between powder particles is increased in size too much, and the densification of the finally obtained sintered compact may be insufficient.
The average particle diameter is obtained by a laser diffraction method as a particle diameter when the cumulative amount of a powder on a volume basis reaches 50%.
In the case where the inorganic powder to be used in the embodiment of the invention is composed of an Fe-based alloy, the tap density thereof is preferably 3.5 g/cm³ or more, more preferably 3.8 g/cm³ or more. If the inorganic powder has a high tap density as described above, the filling property in the interspace between the particles when molding is particularly enhanced. Due to this, it is possible to eventually obtain a particularly dense sintered compact. The tap density of the inorganic powder can be measured according to, for example, the method for measuring a tap density specified in JIS Z 2512.
The specific surface area of the inorganic powder to be used in the embodiment of the invention is not particularly limited, but is preferably 0.15 m²/g or more and 0.8 m²/g or less, more preferably 0.2 m²/g or more and 0.7 m²/g or less, further more preferably 0.3 m²/g or more and 0.6 m²/g or less. If the inorganic powder has a large specific surface area as described above, the surface activity (surface energy) is increased, and therefore, sintering can be easily achieved even by applying less energy. Therefore, when a molded body is sintered, the sintering can be achieved in a shorter time, and the shape retainability is easily enhanced. On the other hand, if the specific surface area exceeds the above upper limit, a contact area between the inorganic powder and the binder is increased more than necessary, and the stability and fluidity of the composition for injection molding may be deteriorated. The specific surface area of the inorganic powder can be measured according to, for example, the method for measuring a specific surface area of a powder (solid) by gas adsorption specified in JIS Z 8830.

Binder

The binder to be used in the embodiment of the invention contains, as described above, a polyacetal-based resin as a component A and an ethylene-glycidyl methacrylate-based copolymer as a component B. Hereinafter, the respective components will be described in detail.

Component A

The component A is a polyacetal-based resin. The polyacetal-based resin is a polymer having an oxymethylene structure as a unit structure and is a copolymer containing formaldehyde and a monomer other than formaldehyde, or the like. A copolymer is used for the purposes of enhancement of shape retainability. Examples of the monomer (comonomer) other than formaldehyde in the copolymer include oxyalkylenes such as oxyethylene and oxypropylene, and also include epichlorohydrin, 1,3-dioxolane, diethylene glycol formal, 1,4-butanediol formal, and 1,3-dioxane, and particularly a monomer having an oxyalkylene unit having 2 or more carbon atoms per molecule is preferably used. The copolymerization amount of the comonomer is not particularly limited, but is preferably 1 part by mole or more and 10 parts by mole or less, more preferably 1 part by mole or more and 6 parts by mole or less with respect to 100 parts by mole of the main monomer. The monomer sequence in such a copolymer is not particularly limited, and any of random copolymerization, alternating copolymerization, block copolymerization, and graft copolymerization may be used.
As such a polyacetal-based resin, for example, Duracon manufactured by Polyplastics Co., Ltd., Tenac manufactured by Asahi Kasei Chemicals Corporation, Iupital manufactured by Mitsubishi Engineering-Plastics Corporation, Polypenco Acetal manufactured by Quadrant Polypenco Japan Ltd., Amilus manufactured by Toray Industries, Inc. , or the like can be used.
Further, the component A has a tensile strength of preferably about 30 MPa or more and 90 MPa or less, more preferably about 40 MPa or more and 80 MPa or less. If the component A has a tensile strength within the above range, the shape retainability of the molded body after molding can be particularly enhanced.

Component B

The component B is an ethylene-glycidyl methacrylate-based copolymer. The ethylene-glycidyl methacrylate-based copolymer is a copolymer having an ethylene structure and a glycidyl methacrylate structure as unit structures. Further, the component B may contain, as another comonomer, one or more structures of carboxylic acid vinyl esters such as vinyl acetate, vinyl propionate, methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, and butyl methacrylate; and ethylenically unsaturated ester compounds such as an α,β-unsaturated carboxylic acid alkyl ester. Among these, particularly, the component B preferably contains at least one of vinyl acetate and methyl acrylate. According to this, the composition for injection molding capable of molding a molded body having particularly high shape retainability is obtained, and it is possible to eventually produce a sintered compact which is particularly less deformed, chipped, or the like and has high quality.
Here, as described above, the composition for injection molding of the embodiment of the invention includes a binder containing a polyacetal-based resin as the component A and an ethylene-glycidyl methacrylate-based copolymer as the component B.
The present inventors made intensive studies of the cause of the low shape retainability of a composition for injection molding in the related art when degreasing and the occurrence of a defect such as deformation or chipping after sintering. As a result, they found that the cause of the low shape retainability is that a metallic element in the inorganic powder performs a catalytic function to accelerate the decomposition of the binder, whereby the binder is decomposed more than necessary and the original function of the binder to bind the inorganic powder particles to one another is deteriorated. On the basis of this finding, they found that the above problem can be solved by incorporating both of the components A and B as a binder, and thus completed the invention.
Specifically, when the binder contains the above-described two components, the component B flows at the time of kneading the staring materials and penetrates to cover the inorganic powder particles. This is because since the softening point of the component B is relatively lower than that of the component A, the component B is transformed into a liquid prior to the component A so that the component B can penetrate into an interspace between the inorganic powder particles and the binder. Moreover, since the component B when softening has relatively high fluidity after softening, the component B can efficiently penetrate also into a small space by a capillary phenomenon. As a result, in the kneaded material (the composition for injection molding of the embodiment of the invention) obtained by kneading the starting materials, the component B is present so as to cover the inorganic powder particles, and the component A is present so as to cover the outer side thereof. Further, the significant decomposition of the component A is suppressed and the component A can be gradually decomposed in a degreasing step, and therefore, a decrease in shape retainability is suppressed.
FIG. 1 is a cross-sectional view schematically showing a structure of the composition for injection molding of the embodiment of the invention, and FIG. 2 is a cross-sectional view schematically showing a structure of the composition for injection molding of the embodiment of the invention when kneading.
As shown in FIG. 1, in a composition for injection molding 10, a plurality of inorganic powder particles 2 are dispersed in a binder 3. In the binder 3, a component A and a component B intermingle with each other.
When kneading such a composition for injection molding 10, the temperature of the composition for injection molding 10 is increased by heating from the outside or self-heating accompanying kneading. As a result, as shown in FIG. 2, in the obtained kneaded material 1, an inner layer 21 composed mainly of the component B is formed so as to cover the surface of each particle 2, and an outer layer 22 composed mainly of the component A is located outside the inner layer 21. If such an inner layer 21 is formed, the inner layer 21 serves as a partition and prevents the contact between the metallic element in the particle 2 and the outer layer 22, whereby the above-described catalytic function is suppressed. As a result, the rapid decomposition of the binder 3 is suppressed, and a decrease in shape retainability can be avoided.
Since the inner layer 21 is in contact with the particle 2, the inner layer 21 is required to have an ability to resist the above-described catalytic function, and on the other hand, the outer layer 22 preferably has excellent fluidity although the outer layer 22 has a higher softening point than the inner layer 21. In view of this, the present inventors found that by using a polyacetal-based resin as the component A and an ethylene-glycidyl methacrylate-based copolymer as the component B, the above-described both requirements can be reliably achieved. By providing the inner layer 21, when a molded body is subjected to a degreasing treatment, the outer layer 22 is not rapidly, but gradually decomposed by heat, and therefore, the shape of the molded body is easily maintained. Since the ethylene-glycidyl methacrylate-based copolymer has an excellent blocking property against the catalytic function of the metallic element and excellent compatibility with the polyacetal-based resin, the moldability can be further enhanced. Accordingly, both of the shape retainability and the moldability can be achieved.
Here, the component B contains a glycidyl group. The glycidyl group is ring-opened during kneading, molding, etc., and binds to a hydroxy group on the surfaces of the inorganic powder particles. As a result, high adhesiveness is exhibited between the inorganic powder and the component B, resulting in stably forming the inner layer 21. On the other hand, it is considered that the ethylene structure provides the above-described blocking property and also provides the compatibility with the component A.
When the component B contains a unit structure derived from the above-described ethylenically unsaturated ester compound, this ethylenically unsaturated ester compound more reliably binds the particles 2 to one another, and therefore, the shape retainability can be particularly enhanced.
The thickness of the inner layer 21 is not particularly limited as long as the inner layer 21 covers the surface of the particle 2, however, for example, an average thickness thereof is preferably 1 nm or more and 2000 nm or less, more preferably 2 nm or more and 1000 nm or less. According to this, both of the shape retainability and the moldability (shape transferability) can be highly achieved. If the average thickness of the inner layer 21 is lower than the above lower limit, the inner layer 21 is likely to be discontinuous, and therefore, the particle 2 and the outer layer 22 may come into contact with each other. On the other hand, if the average thickness thereof exceeds the above upper limit, the ratio of the outer layer 22 is relatively decreased, and therefore, the moldability may be deteriorated.
The outer layer 22 may not be in the form of a layer as long as it is located outside the inner layer 21, and may be in the form such that the outer layers 22 associated with the respective particles 2 are connected to one another, i.e., as shown in FIG. 2, in the kneaded material 1, the outer layer 22 may be in the form of a matrix in which the particles 2 are dispersed. Similarly, the inner layers 21 may not be independent of one another with respect to each particle 2, and may be distributed so as to connect the particles 2 to one another.
The inner layer 21 may be mainly composed of the component B, but may contain the component A or another component. Similarly, the outer layer 22 may be mainly composed of the component A, but may contain the component B or another component. The content of the component B in the inner layer 21 may be more than 50% on a mass basis, and similarly, the content of the component A in the outer layer 22 may be more than 50% on a mass basis.
At a boundary between the inner layer 21 and the outer layer 22, the constituent materials may continuously change through the interface, however, it is preferred that the constituent materials discontinuously change. According to such a configuration, the interface between the inner layer 21 and the outer layer 22 serves as a sliding surface, and the fluidity of the composition for injection molding is particularly improved. As a result, the moldability at the time of injection molding is particularly enhanced, and eventually, a sintered compact having high dimensional accuracy is obtained.
The amount of the component B in the composition for injection molding is set to 1% by mass or more and 30% by mass or less, preferably 2% by mass or more and 20% by mass or less with respect to the amount of the component A. By setting the amount of the component B within the above range, both of the shape retainability and the moldability can be highly achieved.
The softening point of the component B is preferably 65°C or higher and 105°C or lower, more preferably 70°C or higher and 100°C or lower. According to this, when kneading or molding the composition for injection molding, the component B can be reliably softened, whereby the inner layer 21 can be formed.
A difference in softening point between the component B and the component A is preferably 55°C or more and 120°C or less, more preferably 60°C or more and 115°C or less. If the difference in softening point between the component B and the component A is within the above range, both of the shape retainability and the moldability can be more highly achieved.
As the unit structures constituting the ethylene-glycidyl methacrylate-based copolymer as the component B, an ethylene structure, a glycidyl methacrylate structure, an ethylenically unsaturated ester compound structure, etc. can be used as described above, and the abundance ratios of these structures are as follows: the ratio of the ethylene structure is about 60 to 99.9% by mass, the ratio of the glycidyl methacrylate structure is about 0.1 to 20% by mass, and the ratio of the ethylenically unsaturated ester compound structure is about 0 to 39.9% by mass.
Particularly, the ratio of the glycidyl methacrylate structure is preferably 1 part by mass or more and 30 parts by mass or less, more preferably 2 parts by mass or more and 25 parts by mass or less with respect to 100 parts by mass of the ethylene structure. According to this, a balance between the compatibility with the component A attributed to the ethylene structure and the affinity for the particle 2 attributed to the glycidyl methacrylate structure can be highly achieved, and therefore, both of the shape retainability and the moldability can be particularly enhanced.
The ratio of the ethylenically unsaturated ester compound structure is preferably 20 parts by mass or more and 80 parts by mass or less, more preferably 25 parts by mass or more and 75 parts by mass or less with respect to 100 parts by mass of the glycidyl methacrylate structure.
The melt flow rate of the component B is preferably about 0.5 g/10 min or more and 50 g/10 min or less, more preferably about 3 g/10 min or more and 40 g/10 min or less. If the melt flow rate is within the above range, the inner layer 21 is reliably formed, and therefore, the shape retainability of the composition for injection molding of the embodiment of the invention is particularly improved. The melt flow rate can be measured at a temperature of 190°C under a load of 2.16 kg according to the method specified in JIS K 6922-2.
The tensile strength of the component B is preferably about 4 MPa or more and 25 MPa or less, more preferably about 5 MPa or more and 20 MPa or less. According to this, the component B has high fluidity also when softening, and therefore, the inner layer 21 can be more reliably formed.
The weight average molecular weight of the component B is appropriately set in consideration of the melt flow rate as described above or the like, however, it is, for example, preferably 10,000 or more and 400,000 or less, more preferably 30,000 or more and 300,000 or less.

Another Component

The binder to be used in the embodiment of the invention may contain another component.
As the another component, a wax is preferably used. Examples of the wax include natural waxes including vegetable waxes such as candelilla wax, carnauba wax, rice wax, Japan wax, and jojoba wax; animal waxes such as beeswax, lanolin, and spermaceti wax; mineral waxes such as montan wax, ozokerite, and ceresin; and petroleum-based waxes such as paraffin wax, microcrystalline wax, and petrolatum; and synthetic waxes including synthetic hydrocarbons such as polyethylene wax; modified waxes such as montan wax derivatives, paraffin wax derivatives, and microcrystalline wax derivatives; hydrogenated waxes such as hydrogenated castor oil and hydrogenated castor oil derivatives; fatty acids such as 12-hydroxystearic acid; acid amides such as stearic acid amide; and esters such as phthalic anhydride imide. Among these waxes, one wax can be used or two or more waxes can be used in combination. By incorporating such a wax, the fluidity of the composition for injection molding is increased, and therefore the moldability thereof is further enhanced.
As the wax, particularly a petroleum-based wax or a modified product thereof is preferably used, and paraffin wax, microcrystalline wax, carnauba wax, or a derivative thereof is more preferably used, and paraffin wax or carnauba wax is further more preferably used. Such a wax has excellent compatibility with the component A, and therefore enables the preparation of a homogeneous binder. Accordingly, such a wax contributes to the production of the composition of injection molding having high moldability.
The weight average molecular weight of the wax is preferably 100 or more and less than 10, 000, more preferably 200 or more and 5,000 or less. By setting the weight average molecular weight of the wax within the above range, the inorganic powder and the binder can be more uniformly mixed, and therefore, the moldability of the composition for injection molding can be further enhanced.
The content of the wax in the binder is preferably 0.1% by mass or more and 20% by mass or less, more preferably 1% by mass or more and 15% by mass or less. By setting the content of the wax within the above range, the fluidity of the composition for injection molding can be particularly increased.
The ratio of the wax to the component B is preferably 0.01 or more and 0.5 or less, more preferably 0.02 or more and 0.4 or less. By setting the ratio of the wax to the component B within the above range, a balance between the component B and the wax is optimized, and therefore, the moldability can be enhanced without impairing the shape retainability when degreasing.
As the wax, a wax having a softening point of 30°C or higher and 200°C or lower is preferably used, and a wax having a softening point of 50°C or higher and 150°C or lower is more preferably used.
Additional examples of the another component include higher fatty acids such as stearic acid, oleic acid, and linoleic acid; higher fatty acid amides such as stearic acid amide, palmitic acid amide, and oleic acid amide; higher alcohols such as stearin alcohol and ethylene glycol; fatty acid esters such as palm oil; phthalic acid esters such as diethyl phthalate and dibutyl phathalate; adipic acid esters such as dibutyl adipate; sebacic acid esters such as dibutyl sebacate; polyvinyl alcohol, polyvinylpyrrolidone, polyether, polypropylene carbonate, ethylenebisstearamide, sodium alginate, agar, gum arabic, resins, sucrose, and ethylene-vinyl acetate copolymers (EVA). Among these components, one component can be used or two or more components can be used in combination.
Among these, as the higher fatty acid, particularly, a saturated fatty acid such as lauric acid, myristic acid, palmitic acid, stearic acid, or arachidic acid is preferably used. Such a saturated fatty acid contains a long-chain alkyl group, but does not contain an unsaturated bond, and therefore functions as a lubricant and can further enhance the moldability of the composition for injection molding.
The content of the saturated fatty acid in the binder is preferably 0.1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 8% by mass or less. By setting the content of the saturated fatty acid within the above range, the moldability of the composition for injection molding can be particularly enhanced.
The ratio of the saturated fatty acid to the component B is preferably 0.01 or more and 1 or less, more preferably 0.02 or more and 0.5 or less. By setting the ratio of the saturated fatty acid to the component B within the above range, both of the shape retainability and the moldability can be more highly achieved.
Further additional examples of the another component include polyolefins such as polyethylene, polypropylene, polybutylene, and polypentene; polyolefin-based copolymers such as a polyethylene-polypropylene copolymer and a polyethylene-polybutylene copolymer; and hydrocarbon-based resins such as polystyrene.
The composition for injection molding may further contain an antioxidant, a degreasing accelerating agent, a surfactant, or the like other than the above-described components.
The content of the binder in the composition for injection molding is appropriately set according to the metal powder or the ceramic powder, however, it is set to preferably about 1 part by mass or more and 50 parts by mass or less, more preferably about 3 parts by mass or more and 30 parts by mass or less with respect to 100 parts by mass of the inorganic powder. According to this, the shape retainability of the composition for injection molding when degreasing is particularly enhanced.
The composition for injection molding is prepared by kneading the above-described inorganic powder, binder, and the like. When kneading these components, any of various kneading machines, for example, a pressure or double-arm kneader-type kneading machine, a roller-type kneading machine, a Banbury-type kneading machine, a single-screw or twin-screw extruding machine, or the like can be used.
The kneading conditions vary depending on the various conditions such as the particle diameter of the metal powder to be used, the mixing ratio of the metal powder to the binder composition, etc., however, for example, the kneading temperature can be set to 50 to 200°C, and the kneading time can be set to 15 to 210 minutes.

Method for Producing Sintered Compact

Hereinafter, the method for producing a sintered compact of the embodiment of the invention will be described.
The method for producing a sintered compact includes: a kneading step in which an inorganic powder and a binder are kneaded, thereby obtaining a kneaded material (the composition for injection molding of the embodiment of the invention); a molding step in which the thus obtained kneaded material is molded into a desired shape; a degreasing step in which the thus obtained molded body is degreased; and a firing step in which the thus obtained degreased body is fired. Hereinafter, the respective steps will be sequentially described.

Kneading Step

The kneaded material is prepared by kneading the above-described inorganic powder, binder, and the like. When kneading these components, any of various kneading machines, for example, a pressure or double-arm kneader-type kneading machine, a roller-type kneading machine, a Banbury-type kneading machine, a single-screw or twin-screw extruding machine, or the like can be used.
The kneading temperature in the kneading step is preferably set according to the softening points of the components A and B. Specifically, since the ethylene-glycidyl methacrylate-based copolymer as the component B has a lower softening point than the polyacetal-based resin as the component A, the initial kneading temperature is preferably set to a temperature between the softening point of the component A and a temperature lower than the softening point of the component A by 10°C. By kneading the components at such a temperature, only the component B is softened as the temperature is raised when kneading so that it becomes easy for the component B to penetrate between the particle 2 and the component A. As a result, the inner layer 21 and the outer layer 22 are formed, whereby both of the shape retainability and the moldability can be highly achieved.
When the softening point of the component A is represented by T_A [°C] and the softening point of the component B is represented by T_B [°C], as described above, the kneading temperature is preferably [T_A-10] °C or higher and T_A°C or lower, more preferably [T_A-10]°C or higher and [T_A-2]°C or lower. By kneading the components at such a temperature, the above-described effect becomes more pronounced. Further, it is preferred that such a kneading temperature is maintained for about 5 minutes or more and 180 minutes or less, with the proviso that T_A and T_B preferably satisfy the relationship: T_A-10 > T_B.
After completing the kneading under the above conditions, kneading may be performed at a temperature higher than the softening point of the component A (T_A) in the end. According to this, also the component A is softened, and the fluidity of the entire kneaded material is further improved. In this case, the final kneading temperature is preferably higher than T_A°C and [T_A+70]°C or lower.
The total kneading time is preferably about 15 minutes or more and 210 minutes or less.
The viscosity of the thus obtained kneaded material is preferably 500 P or more and 7,000 P or less (50 Pa·s or more and 700 Pa·s or less), more preferably 1,000 P or more and 6,000 P or less (100 Pa·s or more and 600 Pa·s or less). According to this, the moldability when molding can be particularly enhanced. The viscosity is measured using a capirograph by maintaining the temperature of the kneaded material at 190°C.
As the binder to be subjected to this step, a binder in the form of a powder is preferably used. When transforming the binder into a powder, a common grinding method is used, however, particularly, cryogenic grinding is preferably used. A binder powder obtained by cryogenic grinding is particularly fine and uniform, and moreover, has the original binder property since a heating effect when grinding is suppressed. Therefore, the above-described effect on the basis of the difference in softening point between the component A and the component B is more reliably exhibited. As a result, the inner layer 21 and the outer layer 22 are reliably formed around the circumference of the inorganic powder particle, whereby a kneaded material which can highly achieve both of the shape retainability and the moldability is obtained.
The cryogenic grinding is a method of finely and uniformly grinding a sample by utilizing the brittleness caused by the freezing of the sample. In the cryogenic grinding, a cryogenic grinding machine is used. The cryogenic grinding machine is provided with a grinding vessel, in which a sample is placed, and steel balls, which reciprocate in the grinding vessel, and by causing the steel balls to reciprocate while cooling the grinding vessel with a cooling agent such as liquid nitrogen, a sample in the grinding vessel is ground. As cooling progresses, the sample becomes brittle, and therefore, a sample with flexibility can also be ground. The above-described cryogenic grinding machine is described as one example, and a cryogenic grinding machine having other structure can also be used.
By subjecting the binder to cryogenic grinding, the binder can be ground finely and uniformly without denaturing the binder. In the case of using a grinding method other than cryogenic grinding, heat is generated in the binder as grinding progresses, and due to this heat, denaturation, melting (softening), or decomposition may occur, however, by using cryogenic grinding, this denaturation, melting, or decomposition can be prevented. As a result, the binder is to be subjected to the following step while maintaining the original property, and therefore, a decrease in shape retainability of a molded body is prevented. Eventually, it is possible to produce a sintered compact which is less deformed, chipped, or the like and has high quality.
When using cryogenic grinding, the resulting powder is fine and has a large specific surface area and also has a high surface activity. Such a powder has high affinity for the inorganic powder, and when mixing the binder powder with the inorganic powder, the powder contributes to the suppression of the occurrence of a problem such as uneven mixing. Accordingly, the use of cryogenic grinding enables the production of a particularly uniform composition for injection molding.
As the cooling agent in cryogenic grinding, other than liquid nitrogen as described above, liquid air, liquid oxygen, dry ice, or the like may be used.
In the case of performing cryogenic grinding, cryogenic grinding may be performed after mixing the components A and B, or the components A and B may be separately subjected to cryogenic grinding, however, from the viewpoint of obtaining a homogeneous kneaded material by uniformly mixing the components A and B, the latter process is preferred.
The thus obtained binder powder has an average particle diameter of preferably about 10 µm or more and 500 µm or less, more preferably about 15 µm or more and 400 µm or less. By grinding the binder through cryogenic grinding to a particle diameter within the above range, the effect of a difference in specific gravity during mixing can be suppressed to minimum, and therefore, the binder powder and the inorganic powder can be uniformly mixed.
Particularly, in the case of grinding the components A and B separately, the average particle diameter of the binder powder of the component A is preferably 3 times or more and 20 times or less, more preferably 7 times or more and 15 times or less larger than that of the inorganic powder. Meanwhile, the average particle diameter of the binder powder of the component B is preferably 3 times or more and 50 times or less, more preferably 5 times or more and 30 times or less larger than that of the inorganic powder. According to this, the binder powder and the inorganic powder can be more uniformly mixed.
The average particle diameter of the binder powder of the component B is preferably 2 times or more and 15 times or less, more preferably 3 times or more and 10 times or less larger than that of the binder powder of the component A. According to this, the binder powder of the component A and the binder powder of the component B can be more uniformly mixed.
The average particle diameter is obtained by a laser diffraction method as a particle diameter when the cumulative amount of a powder on a volume basis reaches 50%.

Molding Step

First, the composition for injection molding of the embodiment of the invention as described above is molded. According to this, a molded body having a desired shape and dimension is produced.
As the molding method, an injection molding method is used. Incidentally, prior to injection molding, the composition for injection molding may be subjected to a pelletization treatment as needed. The pelletization treatment is a treatment in which a compound is ground using a grinding device such as a pelletizer. The thus obtained pellets have an average particle diameter of about 1 mm or more and 10 mm or less.
Subsequently, the thus obtained pellets are placed in an injection molding machine and injected into a mold to effect molding. According to this, a molded body to which the shape of the mold has been transferred is obtained.
The shape and dimension of the molded body to be produced are determined in anticipation of the amount of shrinkage by degreasing and sintering to be performed thereafter.
The resulting molded body may be subjected to post-processing such as mechanical processing or laser processing as needed.
The injection pressure for the composition for injection molding is preferably from about 5 to 500 MPa.
Further, it is preferred that not only the temperature in the molding step, but also the temperature when the composition for injection molding is kneaded (kneading temperature) is set in the same manner as the above-described molding temperature. According to this, the inner layer 21 is formed also at the kneading stage, and therefore, the fluidity of the kneaded material is improved and uniform kneading can be achieved. Accordingly, also the resulting molded body becomes homogeneous, and the shape retainability and the moldability are further improved.

Degreasing Step

Subsequently, the thus obtained molded body is subjected to a degreasing treatment. According to this, the binder contained in the molded body is removed (degreased), whereby a degreased body is obtained.
The component B is decomposed and discharged outside before the component A when degreasing or molding prior to degreasing in many cases. At this time, a flow path is formed in the molded body. In the degreasing step, a decomposed product of the component A is easily discharged through this flow path, and therefore, a degreasing treatment can be performed while preventing the occurrence of a crack or the like in the molded body. As a result, the shape retainability of the molded body (degreased body) can be particularly enhanced.
The degreasing treatment is not particularly limited, but is performed by a heat treatment in an oxidative atmosphere such as oxygen gas or nitric acid gas, and other than this, in a non-oxidative atmosphere, for example, under vacuum or a reduced pressure (for example, 1.33 x 10^-4 Pa or more and 13.3 Pa or less), or in a gas such as nitrogen gas or argon gas.
The treatment temperature in the degreasing step (heat treatment) is not particularly limited, but is preferably 100°C or higher and 750°C or lower, more preferably 150°C or higher and 700°C or lower.
The treatment time (heat treatment time) in the degreasing step (heat treatment) is not particularly limited, but is preferably 0.5 hours or more and 20 hours or less, more preferably 1 hour or more and 10 hours or less.
The degreasing by such a heat treatment may be performed by being divided into a plurality of steps (stages) for various purposes (for example, for the purpose of reducing the degreasing time, etc.). In this case, for example, a method in which degreasing is performed at a low temperature in the former half and at a high temperature in the latter half, a method in which degreasing at a low temperature and degreasing at a high temperature are alternately repeated, or the like can be used.
After the degreasing treatment as described above, the thus obtained degreased body may be subjected to various post-processing treatments for the purpose of, for example, deburring, forming a microstructure such as a groove, etc.
It is not necessary to completely remove the binder in the molded body by the degreasing treatment, and for example, the binder may partially remain therein at the time of completion of the degreasing treatment.

Firing Step

Subsequently, the degreased body having been subjected to the degreasing treatment is fired. According to this, the degreased body is sintered, whereby a sintered compact (the sintered compact of the embodiment of the invention) is obtained.
The firing conditions are not particularly limited, but the firing step is performed by a heat treatment in a non-oxidative atmosphere, for example, under vacuum or a reduced pressure (for example, 1.33 x 10^-4 Pa or more and 133 Pa or less), or in an inert gas such as nitrogen gas or argon gas. According to this, the oxidation of the metal powder can be prevented.
In the case where a metal material is contained in the inorganic powder, it is preferred that when firing, the degreased body is placed in a vessel composed of a metal material of the same type as the metal material contained in the inorganic powder, and the degreased body is fired in such a state. According to this, the metal component in the degreased body is hardly evaporated, and therefore, the metal composition of the finally obtained sintered compact can be prevented from deviating from the intended composition.
As the vessel to be used, not a vessel having an airtight structure, but a vessel having an appropriate pore or aperture is preferred. According to this, the atmosphere in the inside of the vessel is made the same as that in the outside of the vessel, and can be prevented from changing to an undesired atmosphere. Further, it is preferred that there is a sufficient space between the vessel and the degreased body without adhering to each other as much as possible.
The atmosphere in which the firing step is performed may be changed in the course of the firing step. For example, the initial firing atmosphere is set to a reduced pressure atmosphere, and then, the atmosphere can be changed to an inert atmosphere in the course of the firing step.
The firing step may be performed by being divided into two or more stages. According to this, sintering efficiency is improved, and sintering can be achieved in a shorter firing time.
It is preferred that the firing step is performed continuously with the above-described degreasing step. According to this, the degreasing step can also serve as a pre-sintering step, and therefore, preheating is provided for the degreased body and the degreased body can be more reliably sintered.
The firing temperature is appropriately set according to the type of the inorganic powder. However, in the case of the metal powder, the firing temperature is preferably 1,000°C or higher and 1,650°C or lower, more preferably 1,050°C or higher and 1,500°C or lower. Meanwhile in the case of the ceramic powder, the firing temperature is preferably 1,250°C or higher and 1,900°C or lower, more preferably 1,300°C or higher and 1,800°C or lower.
The firing time is preferably 0.5 hours or more and 20 hours or less, more preferably 1 hour or more and 15 hours or less.
Such a firing step may be performed by being divided into a plurality of steps (stages) for various purposes (for example, for the purpose of reducing the firing time, etc.). In this case, for example, a method in which firing is performed at a low temperature in the former half and at a high temperature in the latter half, a method in which firing at a low temperature and firing at a high temperature are alternately repeated, or the like can be used.
After the firing step as described above, the thus obtained sintered compact may be subjected to mechanical processing, electric discharge processing, laser processing, etching, or the like for the purpose of, for example, deburring, forming a microstructure such as a groove, etc.
The obtained sintered compact may be subjected to an HIP treatment (hot isostatic press treatment) etc. as needed. According to this, the density of the sintered compact can be further increased.
As for the conditions for the HIP treatment, for example, the temperature is set to 850°C or higher and 1,100°C or lower, and the time is set to 1 hour or more and 10 hours or less.
Further, the pressure to be applied is preferably 50 MPa or more, more preferably 100 MPa or more.
The sintered compact obtained as described above may be used in any purpose, and as the use thereof, various structural parts, various medical structures, and the like can be exemplified.
The relative density of the thus obtained sintered compact is expected to be, for example, 95% or more, preferably 96% or more. Such a sintered compact has a high sintering density and has excellent appearance and dimensional accuracy.
Further, the tensile strength of the sintered compact is expected to be, for example, 900 MPa or more in the case of using a metal powder. In addition, the 0.2% proof stress of the sintered compact is expected to be, for example, 750 MPa or more in the case of using a metal powder.
Hereinabove, the invention is described based on preferred embodiments, however, the invention is not limited thereto.

Examples

Next, specific Examples of the invention will be described.

1. Production of Sintered Compact

Example 1

First, an SUS316L powder (powder No. 1) produced by a water atomization method was prepared. The average particle diameter of the SUS316L powder was measured using a laser diffraction particle size distribution analyzer (Microtrac HRA 9320-X100, manufactured by Nikkiso Co., Ltd.). The measured values are shown in Table 1.

Table 1

	Formulation	Average particle diameter [µm]	Amount of binder with respect to 100 parts by mass of powder (parts by mass)
Powder No. 1	SUS316L	10	10
Powder No. 2	2%Ni-Fe	6	9
Powder No. 3	Ti-6Al-4V	17	11
Powder No. 4	Alumina	0.5	30

On the other hand, a binder having a formulation shown in Table 2 was prepared, and a component A, a component B, and another component such as a lubricant were separately cryogenically ground. By doing this, a first binder powder obtained by cryogenically grinding the component A, a second binder powder obtained by cryogenically grinding the component B, and a third binder powder obtained by cryogenic grinding the lubricant or the like were separately produced.
Specifically, a starting material such as the component A was placed in a grinding vessel and ground while cooling with liquid nitrogen. The grinding conditions for the cryogenic grinding were set such that the material temperature was -196°C, the grinding machine temperature was -15°C, and the grinding machine rotation speed was 5,200 rpm. The average particle diameters of the obtained first binder powder, second binder powder, and third binder powder were 53 µm, 242 µm, and 200 µm, respectively.
Subsequently, the SUS316L powder and the binder powders were mixed and kneaded using a pressure kneader (kneading machine) at a kneading temperature of 160°C for 30 minutes. This kneading was performed in a nitrogen atmosphere. The mixing ratio of the SUS316L powder and the binder is shown in Table 1.
Subsequently, the thus obtained kneaded material was ground using a pelletizer, whereby pellets having an average particle diameter of 5 mm were obtained.
Then, the thus obtained pellets were molded by an injection molding machine under the molding conditions that the material temperature was 190°C and the injection pressure was 10.8 MPa (110 kgf/cm²). By doing this, a molded body was obtained. The molded body had a cylindrical shape with a diameter of 0.5 mm and a height of 0.5 mm.
Subsequently, the molded body was subjected to a degreasing treatment under the degreasing conditions that the temperature was 500°C, the time was 1 hour, and the atmosphere was nitrogen gas (atmospheric pressure). By doing this, a degreased body was obtained.
Subsequently, the degreased body was subjected to a firing treatment under the firing conditions that the temperature was 1,270°C, the time was 3 hours, and the atmosphere was nitrogen gas (atmospheric pressure). By doing this, a sintered compact was obtained.

Examples 2 to 13

Sintered compacts were obtained in the same manner as in Example 1 except that a binder having a formulation shown in Table 2 was used as the binder. Incidentally, in Example 12, the kneading temperature was set to 155°C. Example 13 does not form part of the invention and is provided for information only.

Comparative Examples 1 to 6

Sintered compacts were obtained in the same manner as in Example 1 except that the powder No. 1 was used as the inorganic powder and a binder having a formulation shown in Table 2 was used as the binder.
The components A and B in Table 2 shown above and Tables 3 to 5 shown below are the following compounds.

Component A

· Tenac HC750: a polyacetal-based copolymer
· Tenac 7520: a polyacetal-based copolymer
· Tenac 7054: a polyacetal-based homopolymer

Component B

· E-GMA-VA: a glycidyl methacrylate structure: 12% by mass, a vinyl acetate structure: 5% by mass, and an ethylene structure: remainder
· E-GMA-MA: a glycidyl methacrylate structure: 3% by mass, a methyl acrylate structure: 27% by mass, and an ethylene structure: remainder
· E-GMA: a glycidyl methacrylate structure: 12% by mass and an ethylene structure: remainder

As for the melt flow rate of the component B, E-GMA-VA, E-GMA-MA, and E-GMA had melt flow rates of 7 g/10 min, 7 g/10 min, and 3 g/10 min, respectively.

Examples 14 to 16

First, a 2% Ni-Fe alloy powder (powder No. 2) produced by a water atomization method was prepared. The average particle diameter of the powder was measured using a laser diffraction particle size distribution analyzer. The measured values are shown in Table 1. The formulation of the 2% Ni-Fe alloy is as follows: C (0.4% by mass or more and 0.6% by mass or less), Si (0.35% by mass or less), Mn (0.8% by mass or less), P (0.03% by mass or less), S (0.045% by mass or less), Ni (1.5% by mass or more and 2.5% by mass or less), Cr (0.2% by mass or less), and Fe (remainder).
Then, sintered compacts were obtained in the same manner as in Example 1 except that a binder having a formulation shown in Table 3 was used as the binder. Incidentally, the kneading conditions were set such that the temperature was 160°C and the time was 30 minutes. Further, the molding conditions were set such that the material temperature was 190°C. Further, the degreasing conditions were set such that the temperature was 600°C, the time was 1 hour, and the atmosphere was nitrogen gas (atmospheric pressure). Further, the firing conditions were set such that the temperature was 1,150°C, the time was 3 hours, and the atmosphere was nitrogen gas (atmospheric pressure). Example 16 does not form part of the invention and is provided for information only.

Comparative Examples 7 to 11

Sintered compacts were obtained in the same manner as in Example 1 except that the powder No. 2 was used as the inorganic powder and a binder having a formulation shown in Table 3 was used as the binder.

Table 3

	Classification	Component	Softening point	Unit	Example 14	Example 15	Example 16**	Comparative Example 7	Comparative Example 8	Comparative Example 9	Comparative Example 10	Comparative Example 11
	Component A	Tenac HC750	170°C	% by mass	88	87		98	92		96.5
		Tenac 7520	160°C	% by mass
		Tenac 7054	165°C	% by mass			83					72
	Component B	E-GMA-VA	95°C	% by mass	10	7	9				0.5
		E-GMA-MA	52°C	% by mass		2
		E-GMA	103°C	% by mass								23
	Wax	Paraffin wax	60°C	% by mass		1	3		5	2	1	2
Binder		Microcrystalline wax	70°C	% by mass		0.5
		Polyethylene wax	110°C	% by mass
	Other	Dibutyl phthalate	-	% by mass		0.5			1
	Other	Stearic acid	70°C	% by mass	2	2	5	2	2	3	2	3
	EVA		45°C	% by mass						40
	Polystyrene		-	% by mass						55
	Component B/Component A x 100		-	% by mass	11.4	10.3	10.8	-	-	-	0.5	31.9
	Wax/Component B		-	-	0.00	0.17	0.33	-	-	-	2.00	0.09
	Stearic acid/Component B		-	-	0.20	0.22	0.56	-	-	-	4.00	0.13
Inorganic powder	Metalpowder		-	-	No.2	No.2	No.2	No.2	No.2	No.2	No.2	No.2
Kneaded material	Viscosity		-	P	3100	3300	4700	7700	7000	6600	4900	1600
Evaluation results of sintered compact	Sintering density		-	-	98.1	98.6	97.0	95.9	96.4	94.1	95.3	94.1
	Appearance		-	-	A	A	B	D	D	D	D	C
	Dimensional accuracy		-	-	A	A	B	D	D	D	C	D

*: E-GMA indicates that an ethylene structure and a glycidyl methacrylate structure are contained.
*: VA indicates that a vinyl acetate structure is contained, and MA indicates that a methyl acrylate structure is contained.
** : Example 16 does not form part of the invention and is provided for information only.

Examples 17 to 19

First, a Ti alloy powder (powder No. 3) produced by a gas atomization method was prepared. The average particle diameter of the powder was measured using a laser diffraction particle size distribution analyzer. The measured values are shown in Table 1.
Then, sintered compacts were obtained in the same manner as in Example 1 except that a binder having a formulation shown in Table 4 was used as the binder. Incidentally, the kneading conditions were set such that the temperature was 160°C and the time was 30 minutes. Further, the molding conditions were set such that the material temperature was 190°C. Further, the degreasing conditions were set such that the temperature was 450°C, the time was 1 hour, and the atmosphere was nitrogen gas (atmospheric pressure). Further, the firing conditions were set such that the temperature was 1,100°C, the time was 3 hours, and the atmosphere was argon gas (reduced pressure: 1.3 kPa). Example 19 does not form part of the invention and is provided for information only.

Comparative Examples 12 to 16

Sintered compacts were obtained in the same manner as in Example 1 except that the powder No. 3 was used as the inorganic powder and a binder having a formulation shown in Table 4 was used as the binder.

Table 4

	Classification	Component	Softening point	Unit	Example 17	Example 18	Example 19**	Comparative Example 12	Comparative Example 13	Comparative Example 14	Comparative Example 15	Comparative Example 16
	Component A	Tenac HC750	170°C	% by mass	88	87		98	92		96.5
		Tenac 7520	160°C	% by mass
		Tenac 7054	165°C	% by mass			89					72
	Component B	E-GMA-VA	95°C	% by mass	10	8	7				0.5
		E-GMA-MA	52°C	% by mass
		E-GMA	103°C	% by mass		2						23
	Wax	Paraffin wax	60°C	% by mass		1			5	2	1	2
Binder		Microcrystalline wax	70°C	% by mass		0.5	3
		Polyethylene wax	110°C	% by mass
	Other	Dibutyl phthalate	-	% by mass		0.5			1
	Other	Stearic acid	70°C	% by mass	2	1	1	2	2	3	2	3
	EVA		45°C	% by mass						40
	Polystyrene		-	% by mass						55
	Component B/Component A x 100		-	% by mass	11.4	11.5	7.9	-	-	-	0.5	31.9
	Wax/Component B		-	-	0.00	0.15	0.43	-	-	-	2.00	0.09
	Stearic acid/Component B		-	-	0.20	0.10	0.14	-	-	-	4.00	0.13
Inorganic powder	Metal powder		-	-	No.3	No.3	No.3	No.3	No.3	No.3	No.3	No.3
Kneaded material	Viscosity		-	P	3200	3500	4800	7400	7200	6700	5100	6500
Evaluation results of sintered compact	Sintering density		-	-	97.8	98.3	97.2	95.6	96.1	93.8	95.8	94.2
	Appearance		-	-	A	A	B	D	D	D	D	C
	Dimensional accuracy		-	-	A	A	B	D	D	D	C	D

*: E-GMA indicates that an ethylene structure and a glycidyl methacrylate structure are contained.
*: VA indicates that a vinyl acetate structure is contained, and MA indicates that a methyl acrylate structure is contained.
** : Example 19 does not form part of the invention and is provided for information only.

Examples 20 to 22

First, an alumina powder (powder No. 4) was prepared, and the average particle diameter of the powder was measured using a laser diffraction particle size distribution analyzer. The measured values are shown in Table 1.
Then, sintered compacts were obtained in the same manner as in Example 1 except that a binder having a formulation shown in Table 5 was used as the binder. Incidentally, the kneading conditions were set such that the temperature was 160°C and the time was 30 minutes. Further, the molding conditions were set such that the material temperature was 190°C. Further, the degreasing conditions were set such that the temperature was 500°C, the time was 2 hours, and the atmosphere was nitrogen gas (atmospheric pressure). Further, the firing conditions were set such that the temperature was 1,600°C, the time was 3 hours, and the atmosphere was air. Example 22 does not form part of the invention and is provided for information only.

Comparative Examples 17 to 21

Sintered compacts were obtained in the same manner as in Example 1 except that the powder No. 4 was used as the inorganic powder and a binder having a formulation shown in Table 5 was used as the binder.

Table 5

	Classification	Component	Softening point	Unit	Example 20	Example 21	Example 22**	Comparative Example 17	Comparative Example 18	Comparative Example 19	Comparative Example 20	Comparative Example 21
	Component A	Tenac HC750	170°C	% by mass	88	85		98	92		96.5
		Tenac 7520	160°C	% by mass
		Tenac 7054	165°C	% by mass			85					72
	Component B	E-GMA-VA	95°C	% by mass	10	10					0.5
		E-GMA-MA	52°C	% by mass		1	3
		E-GMA	103°C	% by mass			7					23
	Wax	Paraffin wax	60°C	% by mass		1	2		5	2	1	2
Binder		Microcrystalline wax	70°C	% by mass		1
		Polyethylene wax	110°C	% by mass
	Other	Dibutyl phthalate	-	% by mass		0.5			1
	Other	Stearic acid	70°C	% by mass	2	1.5	3	2	2	3	2	3
	EVA		45°C	% by mass						40
	Polystyrene		-	% by mass						55
	Component B/Component A x 100		-	% by mass	11.4	12.9	11.8	-	-	-	0.5	31.9
	Wax/Component B		-	-	0.00	0.18	0.20	-	-	-	2.00	0.09
	Stearic acid/Component		-	-	0.20	0.14	0.30	-	-	-	4.00	0.13
Inorganic powder	Ceramic powder		-	-	No.4	No.4	No.4	No.4	No.4	No.4	No.4	No.4
material	Viscosity		-	P	3500	3700	4700	7800	7600	8000	7200	1300
Evaluation results of sintered compact	Sintering density		-	-	97.2	97.5	96.8	93.1	93.4	92.3	93.5	93.8
	Appearance		-	-	A	A	B	D	D	D	D	C
	Dimensional accuracy		-	-	A	A	B	D	D	D	C	D

*: E-GMA indicates that an ethylene structure and a glycidyl methacrylate structure are contained.
*: VA indicates that a vinyl acetate structure is contained, and MA indicates that a methyl acrylate structure is contained.
** : Example 22 does not form part of the invention and is provided for information only.

2. Evaluation of Kneaded Material

2.1 Evaluation of Viscosity of Kneaded Material

Each of the kneaded materials obtained in Examples and Comparative Examples was maintained at a temperature of 190°C, and the viscosity thereof was measured using a capirograph. The measurement results are shown in Tables 2 to 5.

2.2 Evaluation by Microscopic Observation

Each of the kneaded materials obtained in Examples and Comparative Examples was placed in fuming nitric acid at 120°C for 3 hours, whereby the component A was selectively removed from the kneaded material. The polyacetal-based resin as the component A is decomposed at a temperature lower than the softening point in fuming nitric acid, and therefore can be selectively removed. Accordingly, by performing this treatment, the outer layer 22 can be selectively removed from the kneaded material. As a result, in the kneaded material, the inorganic powder and the inner layer 21 mainly remain.
Then, the kneaded material subjected to the fuming nitric acid treatment was observed by a scanning electron microscope. In FIGS. 3A and 3B, observed images of the kneaded materials obtained in Example 4 and Comparative Example 2 are shown as representatives, respectively.
As shown in FIG. 3A, in the case of the kneaded material obtained in Example 4 subjected to the fuming nitric acid treatment, a state in which the inner layer 21 is present so as to connect the inorganic powder particles to one another is observed. Further, it is observed that the surface of a substance which looks like a particle has relatively high smoothness. Accordingly, it is confirmed that the inorganic powder particles shown in FIG. 3A are covered with the inner layer 21 without any uncovered areas.
On the other hand, as shown in FIG. 3B, in the case of the kneaded material obtained in Comparative Example 2 subjected to the fuming nitric acid treatment, the inner layer 21 which is present so as to connect the inorganic powder particles to one another is almost not observed. Further, it is observed that on the surface of a substance which looks like a particle, a difference between light and shade is large, and the surface has relatively low smoothness. Accordingly, it is confirmed that on the surfaces of the inorganic powder particles shown in FIG. 3B, even if the inner layer 21 is present, uncovered areas are present.
Incidentally, a qualitative analysis was performed for the kneaded material obtained in Example 4 subjected to the fuming nitric acid treatment by a Fourier transform infrared spectrophotometer (FT-IR). As a result, a spectrum showing characteristics derived from bonds contained mainly in the component B was obtained.
From the results, it is confirmed that in each Example, the inner layer 21 and the outer layer 22 are reliably formed.

3. Evaluation of Sintered Compact

3.1 Evaluation of Sintering Density

The density of each of the sintered compacts obtained in Examples and Comparative Examples was measured by a method according to the Archimedean method (specified in JIS Z 2501) . Further, from the measured sintering density and the true density of the inorganic powder, the relative density of the sintered compact was calculated.

3.2 Evaluation of Appearance

The appearance was evaluated according to the following evaluation criteria by observing 100 sintered compacts obtained in each of Examples and Comparative Examples.

Evaluation Criteria for Appearance

A: The number of sintered compacts in which cracking, chipping, or deformation occurred is 3 or less.
B: The number of sintered compacts in which cracking, chipping, or deformation occurred is 4 or more and 10 or less.
C: The number of sintered compacts in which cracking, chipping, or deformation occurred is 11 or more and 50 or less.
D: The number of sintered compacts in which cracking, chipping, or deformation occurred is 51 or more.

3.3 Evaluation of Dimensional Accuracy

The diameters of 100 sintered compacts obtained in each of Examples and Comparative Examples were measured by a micrometer. Then, for the measured values, evaluation was performed according to the following evaluation criteria based on the "Permissible Deviations in Dimensions Without Tolerance Indication for Widths" specified in JIS B 0411 (Permissible Deviations in Dimensions Without Tolerance Indication for Metallic Sintered Products).

Evaluation Criteria for Dimensional Accuracy

A: Grade is fine (tolerance is ±0.05 mm or less)
B: Grade is medium (tolerance exceeds ±0.05 mm but is ±0.1 mm or less)
C: Grade is coarse (tolerance exceeds ±0.1 mm but is ±0.2 mm or less)
D: Outside the permissible tolerance

The evaluation results of the items 2 and 3 are shown in Tables 2 to 5.
As apparent from Tables 2 to 5, it was confirmed that the respective sintered compacts obtained in Examples have a higher sintering density than the respective sintered compacts obtained in Comparative Examples. Further, it was confirmed that the respective sintered compacts obtained in Examples have superior appearance and dimensional accuracy to the respective sintered compacts obtained in Comparative Examples.

4. Evaluation of Sample for Evaluation

4.1 Production of Sample for Evaluation

First, in order to clarify the relationship between the grinding conditions and the state of the kneaded material, by using a binder powder and an inorganic powder, each of which was ground under the following grinding conditions, a kneaded material as a sample for evaluation was produced. As the inorganic powder and the binder powder, the same powders as in Example 3 were used, and kneading was performed under the same conditions as in Example 3, whereby the kneaded material was obtained.

4.2 Evaluation of Viscosity of Sample for Evaluation

Subsequently, the thus produced sample for evaluation was maintained at a temperature of 190°C, and the viscosity thereof was measured using a capirograph. Then, the viscosity was evaluated according to the following evaluation criteria.

Evaluation Criteria for Viscosity

A: The viscosity is within a range in which both of the moldability and the shape retainability can be enhanced.
B: The viscosity is within a range in which the shape retainability is high but the moldability is slightly poor.
C: The viscosity is within a range in which both of the moldability and the shape retainability are poor.

4.3 Evaluation of Sample for Evaluation by Microscopic Observation

Subsequently, the thus produced sample for evaluation was subjected to the above-described fuming nitric acid treatment, and the outer layer 22 was selectively removed from each sample for evaluation.
Then, the remainder was observed by a scanning electron microscope, and an observed image was obtained.

Evaluation Criteria for Microscopically Observed Image

A: A lot of necks are observed (necks are present in 70% or more of the interspaces between particulate substances).
B: A few necks are observed (necks are present in 20% or more and less than 70% of the interspaces between particulate substances).
C: Necks are not observed (necks are present in less than 20% of the interspaces between particulate substances).

The evaluation results of the items 4.2 and 4.3 are shown in Table 6. Incidentally, the term "neck" refers to a substance which is present so as to connect particulate substances to each other.

Table 6

	Material temperature when grinding [°C]	Grinding machine temperature [°C]	Grinding machine rotation speed [rpm]	Average particle diameter [µm]	Evaluation results of viscosity	Evaluation results of microscopically observed image
Sample 1	-196	-20	3900	67	B	B
Sample 2	-196	-15	5200	53	A	A
Sample 3	20	22	8000	55	C	c

As apparent from Table 6, it was confirmed that the samples 1 and 2 using a powder obtained by cryogenic grinding as the binder powder each had a viscosity suitable for shape retainability, and also in the samples 1 and 2, an inner layer which covers the inorganic powder particle was formed.
On the other hand, the sample 3 using a powder obtained by grinding at normal temperature as the binder powder had low shape retainability, and when the microscopic observation was performed, an inner layer which covers the inorganic powder particle was not observed.
From the above results, it was confirmed that by using a binder powder obtained by cryogenic grinding and also by optimizing the grinding machine rotation speed and the average particle diameter, a composition for injection molding capable of forming a molded body having higher shape retainability can be produced.

Claims

A composition for injection molding, comprising:
an inorganic powder composed of a metal material and/or a ceramic material; and

a binder comprising a polyacetal-based resin and an ethylene-glycidyl methacrylate-based copolymer, wherein the polyacetal-based resin is a copolymer, and

the ethylene-glycidyl methacrylate-based copolymer is comprised in an amount of 1 to 30% by mass with respect to the amount of the polyacetal-based resin.
The composition for injection molding according to claim 1, wherein the composition comprises layered particles comprising (i) an inner layer, which is composed mainly of the ethylene-glycidyl methacrylate-based copolymer and is provided so as to cover the surface of each particle of the inorganic powder, and (ii) an outer layer, which is composed mainly of the polyacetal-based resin and is located outside the inner layer.
The composition for injection molding according to claim 1, wherein the ethylene-glycidyl methacrylate-based copolymer comprises at least one of vinyl acetate and methyl acrylate as a monomer constituting the copolymer.
The composition for injection molding according to claim 1, wherein the softening point of the ethylene-glycidyl methacrylate-based copolymer is 65°C to 105°C.
The composition for injection molding according to claim 1, further comprising a saturated fatty acid.
The composition for injection molding according to claim 1, further comprising a wax.
A sintered compact, which is produced using the composition for injection molding according to any of claims 1-6.
A method for producing a sintered compact, comprising:
Kneading (i) an inorganic powder composed of a metal material and/or a ceramic material and (ii) a binder comprising a polyacetal-based resin and an ethylene-glycidyl methacrylate-based copolymer, wherein the polyacetal-based resin is a copolymer, and the ethylene-glycidyl methacrylate-based copolymer is comprised in an amount of 1 to 30% by mass with respect to the amount of the polyacetal-based resin

at a temperature between the softening point of the polyacetal-based resin and a temperature lower than the softening point of the polyacetal-based resin by 10°C, thereby obtaining a kneaded material;

molding the kneaded material, thereby obtaining a molded body; and

degreasing the molded body, followed by firing, thereby obtaining a sintered compact.